A151969 a(n) = smallest integer >= n which has only prime factors 2 and 5.

1, 2, 4, 4, 5, 8, 8, 8, 10, 10, 16, 16, 16, 16, 16, 16, 20, 20, 20, 20, 25, 25, 25, 25, 25, 32, 32, 32, 32, 32, 32, 32, 40, 40, 40, 40, 40, 40, 40, 40, 50, 50, 50, 50, 50, 50, 50, 50, 50, 50, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 80, 80, 80, 80, 80

Offset: 1

Author

N. J. A. Sloane, Aug 23 2009

Keywords

• nonn

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A130916, A134361, A151970.

Programs

Mathematica

Module[{upto=100,pwr,f25},pwr=Ceiling[Log[2,upto]];f25=Select[ Sort[ Flatten[ Table[Times@@@Union[Sort/@Tuples[{2,5},n]],{n,0,pwr}]]],#<= upto&]; Table[SelectFirst[f25,#>i&],{i,0,upto-1}]] (* The program uses the SelectFirst function from Mathematica version 10 *) (* Harvey P. Dale, Jan 28 2016 *)

R

f <- function(n) nextn(n, factors = c(2,5))
a <- matrix(1:256,ncol=1)
apply(a,1,f)